Torque limiting mechanism

ABSTRACT

A high-precision torque limiting mechanism capable of limiting an output torque to a magnitude of not more than a prescribed value, as well as to reduce a frictional torque produced upon occurrence of a slip. To accomplish this object, a ball ramp section ( 13 ) is attached to a slip clutch section ( 3 ) for reducing friction between friction plates ( 31 ) and ( 32 ) forming the slip clutch section ( 3 ) and limiting a torque to be transmitted by utilizing an axial thrust produced at the ball ramp section ( 13 ) in accordance with the magnitude of an input torque. The precision with which the torque to be transmitted is limited is enhanced by making improvements to the shape of an internal side surface of a recess (G) formed in each of cam elements ( 131 ) and ( 132 ) of the ball ramp section ( 13 ).

TECHNICAL FIELD

The present invention relates to power transmission systems ofaircrafts, general industrial machines and the like and, moreparticularly, to a torque limiting mechanism for limiting transmissionof an excessive input torque when such an excessive input torque works.

BACKGROUND ART

A slip clutch is well-known as a mechanism capable of transmitting atorque from an input shaft to an output shaft while limitingtransmission of an excessive input torque when such an excessive inputtorque works (see patent documents noted below for example). The slipclutch is configured to transmit a torque through friction between afriction plate spline-coupled to the input shaft and a friction platespline-coupled to the output shaft and limit transmission of the torqueby causing the friction plates to slip relative to each other when anexcessive input torque works.

Presently, however, a problem exists that it is difficult to reliablylimit an output torque to a desired magnitude of not more than aprescribed value because of instability in friction coefficient anddepending on some operating conditions even when braking materialshaving preferable properties are combined with each other. Particularly,the slip clutch undeniably allows a surge or overshoot phenomenon suchthat the output torque exceeds the prescribed value undesirably to occurbefore and after occurrence of a slip.

In addition, there exists a request to make a frictional torque producedby the friction plates upon occurrence of the slip as small as possible.This is because generation of heat and the amount of wear increase asthe frictional torque produced upon occurrence of the slip increases.

An intended object of the present invention having been made in view ofthe foregoing circumstances is to realize a high-precision torquelimiting mechanism capable of limiting the output torque to a magnitudeof not more than the prescribed value, as well as to reduce thefrictional torque produced upon occurrence of the slip.

-   Patent Document 1: Japanese Patent Application No. 02005-327332-   Patent Document 2: Japanese Patent Laid-Open Publication No.    2002-031156

DISCLOSURE OF THE INVENTION

According to the present invention, a ball ramp section is attached to atorque limiting mechanism having a slip clutch section interveningbetween an input shaft and an output shaft for reducing friction betweenfriction plates forming the slip clutch section by transmitting an inputtorque at least partially to one cam element of the ball ramp sectionand utilizing an axial thrust produced at the ball ramp section inaccordance with a magnitude of the torque thus transmitted. In general,friction plates forming a slip clutch are elastically biased so as to bepressed against each other. When an excessive torque having a magnitudeexceeding a prescribed value works, the torque limiting mechanismreduces the frictional torque produced by the friction plates by causingthe ball ramp section to produce an axial thrust of a required magnitudeand balancing the axial thrust against the elastically biasing force.

Further, the torque limiting mechanism has an arrangement wherein thecam elements of the ball ramp section are formed with respectiverecesses each having an internal side surface shaped such that: theinternal side surface has first and second contact positions; a ballshifts from the first contact position to the second contact position asthe two cam elements rotate relative to each other; and an acute angleformed between a contact surface contacting the ball at the secondcontact position and a plane perpendicular to an axis is smaller than anacute angle formed between a contact surface contacting the ball at thefirst contact position and the plane perpendicular to the axis. When atorque transmitted to the friction plates exceeds a frictional torqueproduced by the friction plates, a slip occurs in the slip clutchsection. An increase in the torque transmitted to the friction platesand a decrease in the frictional torque produced by the friction platesare both caused by an increase in an input torque inputted to the inputshaft. For this reason, even when the ball ramp section is caused tostart operating at the time an input torque of the prescribed valueworks, namely, even when the ball ramp section produces an axial thrustto cause the two cam elements to move away from each other at that time,it is possible that the decrease in frictional torque fails to catch upa further increase in input torque, resulting in occurrence of a slip ata higher frictional torque than estimated. Stated otherwise, an outputtorque having a magnitude exceeding the prescribed value may betransmitted to the output shaft undesirably. The present inventionfocuses attention on the fact that the relationship between the increasein input torque and the increase in axial thrust is determined by theshape of the internal side surface of the recess of each cam element,particularly, the angle of inclination of a contact surface at which theinternal side surface of the recess contacts the ball. Thus, by shapingthe internal side surface of each recess as described above, a moreincrease in axial thrust can be obtained from a less increase in inputtorque after the ball ramp section has started operating.

According to the present invention, it is possible to realize ahigh-precision torque limiting mechanism capable of limiting the outputtorque to a magnitude of not more than the prescribed value, as well asto reduce the frictional torque produced upon occurrence of the slip.

BRIEF OF DRAWINGS

FIG. 1 is an end view showing a torque limiting mechanism according toone embodiment of the present invention;

FIG. 2 is a sectional view showing a portion of concern of a ball rampsection of the torque limiting mechanism;

FIG. 3 is a fragmentary enlarged view of the portion shown in FIG. 2;

FIG. 4 is a graph plotting the relationship between the frictionaltorque and the amount of displacement of a float block and therelationship between the input torque and the amount of displacement ofthe float block; and

FIG. 5 is a cross-sectional view taken along the line 5-5 in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one embodiment of the present invention will be describedwith reference to the drawings. As shown in FIG. 1, a torque limitingmechanism according to the present embodiment includes an input shaft(or output shaft) 1 and an output shaft (or input shaft) 2 which aredriven to rotate about the same axis A, and a slip clutch section 3intervening between the input shaft 1 and the output shaft 2 tointerconnect the two shafts for transmitting a torque inputted to theinput shaft 1 to the output shaft 2.

The input shaft 1 includes an input shaft body 11, an input-side rotormember 12 driven by the input shaft body 11, and a ball ramp section 13intervening between the input shaft body 11 and the rotor member 12. Therotor member 12 is displaceable along the axis A independently to theinput shaft body 11 and rotatable about the axis A. The ball rampsection 13 includes cam elements 131 and 132 forming a pair along theaxis A, and a ball 133 held between recesses (grooves) G each formed inrespective one of the two cam elements 131 and 132 and functions totransmit the input torque to the rotor member 12 as well as to drive afloat block 332 to be described later by producing an axial thrust whenthe input torque having a magnitude exceeding a prescribed value works.In the present embodiment, one cam element 131 is securely fixed to apredetermined portion of the input shaft body 11 in such a manner as toexpand radially, while the other cam element 132 is spline-coupled(key-coupled) to the outer periphery of the rotor member 12 so as toface the cam element 131. In principle, the cam element 132 fails to bedisplaced relative to the rotor member 12.

The slip clutch section 3 includes friction plates 31 positioned closerto the input shaft 1, friction plates 32 positioned closer to the outputshaft 2, and elastically biasing means 33 for pressing the frictionplates 31 and 32 along the axis A. In the present embodiment, theinput-side rotor member 12 is accommodated in the output shaft 2, andthe friction plates 31 and the friction plates 32 are spline-coupled tothe outer periphery of the rotor member 12 and the inner periphery ofthe output shaft 2, respectively, and are interleaved with each otheralong the axis A. The elastically biasing means 33 comprises, aselements thereof, a push block 331 holding the friction plates 31 and 32from one side, the float block 332 pressing the friction plates 31 and32 from the other side, a preload spring 333 elastically biasing thefloat block 332 toward the push block 331, and a brake spring 334intervening between the float block 332 (or push block 331) and thefriction plates 31 and 32 to elastically bias the friction plates 31 and32 toward the push block 331 (or float block 332). The preload spring333 serves also as means for elastically biasing the rotor member 12 andthe cam element 132 toward the cam member 131 via the float block 332.

In FIG. 1, reference numerals 41,42, 43 and 44 designate bearings.

When a torque is inputted to the input shaft 1, the torque istransmitted to the output shaft 2 through friction between the frictionplates 31 and 32, so that the input shaft 1 and the output shaft 2rotate synchronously. When an excessive input torque having a magnitudeexceeding the prescribed value works, the torque to be transmitted islimited by causing the friction plates 31 and 32 to slip relative toeach other. At that time, the ball ramp section 13 is operated toproduce an axial thrust which displaces the input-side rotor member 12and the float block 332 away from the push block 331 along the axis A.By so doing, the biasing force pressing the friction plates 31 and 32 isweakened to reduce the frictional torque produced by the friction plates31 and 32. It is needless to say that the input shaft 1 and the outputshaft 2 rotate asynchronously with each other when the slip occurs.

Further, the present torque limiting device enhances the precision withwhich the torque to be transmitted is limited by making improvements tothe shape of an internal side surface of the recess G formed in each ofthe two cam elements 131 and 132 of the ball ramp section 13. The shapeof the internal side surface of the recess G is shown in FIGS. 2 and 3.The internal side surface of the recess G of each of the cam elements131 and 132 is a continuous surface having plural contact positions G1and G2 for contacting the ball 133. During a phase in which the inputtorque has a magnitude of not more than the prescribed value and theoperation of the ball ramp section 13 is not started yet, no phasedifference occurs between the cam elements 132 and 131 in the directionof rotation and, hence, the cam element 132 is in a position closest tothe cam element 131. In this condition, the ball 133 contacts each ofthe cam elements 131 and 132 at a point adjacent the first contactposition G1. When the ball ramp section 13 is caused to start operatingdue to the magnitude of the input torque exceeding the prescribed value,a phase difference occurs between the cam elements 132 and 131 in thedirection of rotation, so that the cam element 132 is displaced in adirection away from the cam element 131. At that time, the contactposition of the ball 133 is gradually shifted from the point adjacentthe first contact position G1 toward the second contact position G2 andthen toward a point beyond the second contact position G2 in accordancewith the magnitude of the input torque transmitted to the cam element131.

As apparent from FIG. 3, the relationship between a torque T and anaxial thrust F at a position of contact between the internal sidesurface of the recess G and the ball 133 is tan θ≈F/T, where θrepresents an angle of inclination (acute angle) formed between acontact surface S of the internal side surface of the recess G whichcontacts the ball 133 and an imaginary plane B perpendicular to the axisA. Since the cam element 132 is elastically biased by the preload spring333, the amounts of displacement of the cam element 132 and input-siderotor member 12 along the axis A as well as the amount of displacementof the float block 332 along the axis A are substantially proportionalto the axial thrust F. On condition that the torque T is constant, theamount of displacement of the float block 332 along the axis A issubstantially proportional to the angle θ of inclination of the contactsurface S.

In the present torque limiting device, the internal side surface of eachrecess G is shaped so that the angle of inclination of the contactsurface at the second contact position G2 is smaller than the angle ofinclination of the contact surface at the first contact position G1. Inaddition, a surface portion extending from the first contact position G1to the second contact position G2 is shaped into a curved surface ofwhich the angle of inclination decreases gradually. A surface portionextending beyond the second contact position G2 is shaped into aninclined surface having an angle of inclination substantially equal tothe angle of inclination at the second contact position G2 for example.

FIG. 4 plots relationship X between the amount of displacement of thefloat block 332 along the axis A and the frictional torque produced bythe friction plates 31 and 32 and relationship Y between the amount ofdisplacement of the float block 332 and the input torque causing thedisplacement to occur. The frictional torque produced by the frictionplates 31 and 32 is substantially inversely proportional to the amountof displacement of the float block 332. On the other hand, the inputtorque causing the float block 332 to be displaced decreases graduallyfrom the moment at which the ball ramp section 13 starts operating(i.e., the amount of displacement=+0) while increasing the amount ofdisplacement. Such a gradual decrease continues until the ball 133having left the first contact position G1 reaches the second contactposition G2. After the ball 133 has reached the second contact positionG2, the input torque and the amount of displacement are renderedsubstantially proportional to each other by the constant angle ofinclination of the internal side surface.

A slip of the friction plates 31 and 32 relative to each other occurs atthe time the input torque exceeds the frictional torque, namely, at thepoint of intersection of the plots X and Y in FIG. 4. Upon occurrence ofthe slip, the magnitude of the input torque is Ys and the magnitude ofthe frictional torque is Xs. In this way, the present torque limitingdevice makes it possible to reliably limit the torque to be transmittedfrom the input shaft 1 to the output shaft 2 to a magnitude of not morethan the prescribed value, as well as to reduce the frictional torqueproduced by the friction plates 31 and 32 upon occurrence of the slip.

Conventionally known ball ramps, in general, are of the type providedwith recesses each having an internal side surface formed of an inclinedsurface at a constant angle of inclination. If an attempt is made toincrease the axial thrust by such a ball ramp in order to cause thefriction plates to slip relative to each other, the input torque has tobe increased gradually as shown by plot Z in FIG. 4. Therefore, apossibility that an excessive torque having a magnitude exceeding theprescribed value is transmitted undesirably, cannot be denied. Further,the frictional torque produced upon occurrence of a slip increasesinevitably.

The present invention is not limited to the foregoing embodimentspecifically described above. In particular, the shape of the internalside surface of the recess G of each of the cam elements 131 and 132 isnot limited to the example shown.

Specific structures of other portions are not limited to the foregoingembodiment, but may be varied variously without departing from theconcept of the present invention.

1. A torque limiting mechanism having a slip clutch section interveningbetween an input shaft and an output shaft for limiting an output torqueby causing a slip to occur in the slip clutch section when an inputtorque having a magnitude exceeding a prescribed value works,characterized in that: a ball ramp section including a pair of camelements formed with respective recesses and a ball held between therecesses of the pair of cam elements is provided for reducing frictionbetween friction plates forming the slip clutch section by transmittingthe input torque at least partially to one of the cam elements andutilizing an axial thrust produced by rotation of that cam elementrelative to the other cam element in accordance with a magnitude of thetorque thus transmitted; and the recesses of the cam elements each havean internal side surface shaped such that: the internal side surface hasfirst and second contact positions; the ball shifts from the firstcontact position to the second contact position as the two cam elementsrotate relative to each other; and an acute angle formed between acontact surface contacting the ball at the second contact position and aplane perpendicular to an axis is smaller than an acute angle formedbetween a contact surface contacting the ball at the first contactposition and the plane perpendicular to the axis.